1. Field of the Invention
The present invention relates to a molten carbonate fuel cell and, more particularly, to a molten carbonate fuel cell in which an electrolyte body sandwiched between a pair of electrodes is improved.
2. Description of the Related Art
A basic structure of a molten carbonate fuel cell is shown in FIG. 1. An electrolyte body 3 retaining an electrolyte consisting of an alkali carbonate is sandwiched between an anode (fuel electrode) 1 and a cathode (air electrode) 2 which serve as a pair of electrodes. Two housings 4a and 4b abut against peripheral portions of both surfaces of the electrolyte body 3. The anode 1 and the cathode 2 are stored in the housings 4a and 4b, respectively. A supply port 6 for supplying fuel gas (H.sub.2 and CO.sub.2) to the anode 1 and an exhaust port 7 for exhausting exhaust gas (CO.sub.2 and H.sub.2 O) from the anode 1 are formed in the housing 4a in which the anode 1 is arranged. A supply port 8 for supplying an oxidant gas (air and CO.sub.2) to the cathode 2 and an exhaust port 9 for exhausting an exhaust gas (N.sub.2) from the cathode 2 are formed in the housing 4b in which the cathode 2 is arranged.
In the molten carbonate fuel cell shown in FIG. 1, an alkali carbonate mixture in the electrolyte body 3 is melted at a high temperature. The fuel gas (H.sub.2 and CO.sub.2) is supplied to the anode 1 through the supply port 6 of the housing 4a, while the oxidant gas (air and CO.sub.2) is supplied to the cathode 2 through the supply port 8 of the housing 4b, thereby causing a reaction represented by formula (1) at the anode 1 and a reaction represented by formula (2) at the cathode 2: EQU H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e.sup.-( 1) EQU 1/20.sub.2 +CO.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.2- ( 2)
As described above, the electrolyte is melted during operation of the cell. To prevent the molten electrolyte from flowing to the outside, the electrolyte body 3 is formed of a porous body of particles (to be referred to as retaining particles hereinafter). The porous body is impregnated in general with an electrolyte consisting of an alkali carbonate mixture containing at least two carbonates selected from the group consisting of Li.sub.2 CO.sub.3, K.sub.2 CO.sub.3, and Na.sub.2 CO.sub.3. Fine particles selected from the group consisting of .alpha.-LiAlO.sub.2 particles, .beta.-LiAlO.sub.2 particles, and .gamma.-LiAlO.sub.2 particles each having a particle diameter of 0.05 .mu.m to 0.2 .mu.m are used as the retaining particles.
The electrolyte body used in the molten carbonate fuel cell serves not only a medium for migration of carbonate ions (CO.sub.3.sup.2-) but also a gas permeation barrier layer for inhibiting direct mixture (gas crossover) of reaction gases between the anode and the cathode. In order to perform these functions, the electrolyte must be sufficiently retained in the electrolyte body. An outflow of the electrolyte, i.e., electrolyte loss increases an internal resistance and occurrence of a gas crossover.
Japanese Patent Disclosure (Kokai) No. 63-294668 discloses a method of manufacturing an electrolyte body. It is disclosed that retaining particles consisting of .alpha.-LiAlO.sub.2 having a particle diameter of 0.05 .mu.m to 0.2 .mu.m are formed into a porous sheet by using a paper machine, followed by impregnating the porous sheet with an electrolyte consisting of an alkali carbonate mixture so as to obtain a desired electrolyte body. However, the porous body of the electrolyte body is cracked by heat cycles taking place when the operation of the fuel cell is started and stopped, leading to occurrence of a gas crossover.
Japanese Patent Disclosure No. 58-71564 discloses an electrolyte body having a porous body prepared by adding reinforcing particles having a diameter of at least 25 .mu.m to the fine retaining particles noted above. It is also disclosed that the porous body of the electrolyte body can be prepared by adding a long fibrous reinforcing material having a large diameter to the fine retaining particles. However, agglomeration and coalescence take place between the retaining particles and the reinforcing material forming the porous body during operation of the fuel cell, leading to changes in fine structure of the porous body and, thus, to outflow of the electrolyte. As a result, the internal resistance is increased, and a gas crossover is generated. It follows that the life of the fuel cell is shortened.
Japanese Patent Disclosure No. 54-3236 discloses an electrolyte body having a porous body prepared by using fine short fibers having a large specific surface area. However, the porous body fails to exhibit a sufficient mechanical strength because the fibers used are short. In addition, the particular porous body is inferior in its capability of retaining an electrolyte to the porous body made of fine retaining particles.
Further, each of Japanese Patent Disclosure Nos. 58-128670 and 58-129777 discloses an electrolyte body having a porous body prepared by using fine retaining particles and long fibers acting as a reinforcing material. However, the long fibers fail to be mixed uniformly with the fine retaining particles, making it difficult to control as desired the fine structure of the resultant porous body. As a result, large pores tend to be formed in the porous body in the initial stage of operation of the fuel cell comprising the particular electrolyte body. In this case, changes in the fine structure of the porous body are accelerated during operation of the fuel cell, leading to an outflow of the electrolyte impregnated in the porous body in a short time. Further, agglomeration and coalescence take place between the retaining particles and the long fibers forming the porous body during operation of the fuel cell, leading to formation of continuous pores along the long fiber. As a result, a gas crossover is brought about, leading to a shortened life of the fuel cell.